Firstly I would like to say that this is an interesting guide. I like how you explained everything, expressing your point of view. I would like to add something though. 35% magic penetration from Void Staff + Devastating Strikes 6% magic penetration = 41% magic penetration :D. That's not really what I had in mind of telling you, but I had to. Now, you should start experimenting with other items for the time Deathfire Grasp will be removed in 5.2 so you will have time to change your build accordingly. I do not wish to discourage you, I like how you made your build, but Riot are thinking of removing Deathfire Grasp for good, so I thought of telling you this beforehand so your build will not be outdated by 5.2 :).

You cannot max an ability at level eight.

I don't see the difference between choosing a 21/9/0 mastery tree and yours. The additional offense masteries aren't really a great benefit compared to what the defensive ones provide. Nevermind, but the two points in Rc look awful :D

I hate to say this but when preseason ends and season 5 rolls out Deathfire Grasp is going to get removed.

On a different note, excellent job on your guide! I really like the simplistic formatting and detailed descriptions.

A few things to note:
- Have you considered using Doran's Shield as a starting item? It's quite useful if your enemy laner is ranged and likes to autoattack poke a lot.
- On a similar note of being poked, Greater Seal of Armor would seem like a better pick considering you're melee.
- You don't include any other options for items.
- Abyssal Mask + Sorcerer's Shoes + magic pen marks + masteries = 49 flat magic pen
This is enough to melt most adc's, especially if they didn't build Banshee's Veil
- You don't need the AD masteries unless you really suck at last hitting. I get that your ult and w have ad scalings but it adds 14 total AD at level 18.
- You're probably running Feast because you're getting poked to death. Take out the AD and feast for Block and Unyielding . It'll save you quite a bit of autoattack damage, and in addition they block general damage too. Unyielding blocks up to 20% of Malzahar's Malefic Visions. (Believe me, I would know.)

That's just my input on the guide, change what you see fit :P

Once again, good job, and +1!